tudies with the monoclonal antibody (McAb) McAb 33.1 have been roceeding in two directions. The first subject of nvestigation has been the exact nature of the human class II ajor histocompatibility antigen (MHC) recognized by McAb 3.1. It has been shown previously that the antigen is encoded y the HLA-DC subregion, but the degree of heterogeneity of ntigens from this subregion is as yet unclear. McAb 33.1 has een compared to two other DC-specific antibodies, McAb Genox .53 and McAb Leu 10. The investigation has shown quite learly that McAb 33.1 and McAb Leu 10 recognize the same eterminant. McAb Genox 3.53 recognizes a distinct eterminant. Clearance experiments suggested that the Genox .53 determinant is present on only a subset of the 3.1-positive molecules, raising the possibility of molecular eterogeneity. However, structural studies of the separated ools of antigen have failed to demonstrate any primary tructural differences between the Genox 3.53-positive and enox 3.53-negative fractions. The current hypothesis is that he adsorption differences reflect postsynthetic modifications r artifacts of isolation. The second subject under nvestigation is the enhancement of class II gene transcription pon B cell activation. B cells in various states of ctivation are being examined by the technique of Northern lotting to determine the degree of transcription of each ndividual class II MHC gene and to correlate specific nhancement of transcription with specific functional events.